The invention is in the field of using temperature measurements to explore for and characterize hydrocarbon deposits and particularly relates to using temperature measurements taken to a depth of only a few meters at sites arrayed on the floor of a body of water.
It has been known that some oil and gas fields are hotter at the pay depths than the surrounding rocks, as discussed for example in Meyer, H. J. et al., The Relationship of Geothermal Anomalies To Oil And Gas Accumulation In The Rocky Mountain Area, Am. Assn. Petl. Geol. Bulletin, 1983. Meyer et al. report a study of 22 oil and gas fields from six states in the Rocky Mountain region, and conclude that at least 15 of them have positive geothermal anomalies at the pay level, on the basis of drill-stem tests except for a few values from temperature logs and shut-in bottom hole measurements. As to causes of these temperature anomalies, Meyer et al. conclude that upward fluid movement at depth was the most important factor. Similarly, Mufti U.S. Pat. No. 4,120,199 proposes taking thermal gradient measurements in a borehole which does not penetrate a hydrocarbon deposit, and using them as an indication of the proximity of hydrocarbon deposits. The patent suggests that it may be practical in some cases to drill a number of shallow test holes in an area to be explored simply for the purpose of mapping temperature gradients from measurements taken in the 50 to 100 foot depth region, and that even shallower boreholes can be used when a set of readings can be made in such a short time period that seasonal changes can be ignored.
In addition, deep sea floor heat flow measurements have been made using a lance penetrating the sea floor up to a depth of several meters and carrying a few (e.g., six) temperature measuring devices spaced from each other along the lance length. However, it is believed that the temperature gradient measured in shallow water with such a lance cannot be corrected satisfactorily for influences which conceal or obscure the information of interest; it has been discovered as a part of making this invention that at least at water depths of less than a few hundred meters many more temperature measurements per unit length of the lance are needed in order to unscramble the information of interest with sufficient accuracy and reliability.
In view of the known prior art identified above, it is believed that a need exists to find a way to take temperature gradient measurements which do not require the expense of pre-drilling boreholes but which nevertheless contain extractable information which reliably signifies the convection of nearby hydrocarbons, and to process these measurements into useful information on the presence and nature of subsurface hydrocarbons. Important aspects of the invention are directed to meeting that need.
In an exemplary and a nonlimiting embodiment of the invention, a promising subsurface area is selected on the basis of knowledge of its geology from studies such as seismic surveys or from other sources, and suitable information is found as to the surface thermal conductivity of the sea floor material and the long term (e.g., seasonal) bottom water temperature variations. The thermal conductivity of interest can be measured by taking core samples and finding the thermal conductivity by conventional needle probe measurements. Alternately, it can be estimated from otherwise available information on the subsurface lithology. The long term bottom water temperature variations can be measured e.g. with a temperature probe left at the sea bottom in the area of interest and arranged to record the bottom water temperature frequently enough (e.g. hourly, daily or weekly) over a long enough period (e.g. at least three to six months and preferably a year). In the alternative, the seasonal bottom water temperatures can be deduced from sea bottom temperature gradients as discussed below.
An array of measurement sites is selected, preferably on the basis of some knowledge of the subsurface geology, such that there would be no undue repetition of temperature gradients from site to site but, on the other hand, the sites will be close enough to avoid abrupt changes in temperature gradients as between adjacent sites. For example, if the area of interest includes a fault, the sites can be in a line perpendicular to the fault plane and can be more closely spaced at the fault (25-100 meters) and less closely spaced elsewhere (at 1 km intervals). If the subsurface formation includes a symmetrical salt dome, the measurement sites can be on a cross or a regular grid and closely spaced at the dome edges (25-100 m) and less closely spaced away from the dome edges (0.5 km). A special lance, capable of penetrating the sea floor at the area of interest and having a string of closely spaced temperature measuring devices (e.g. 25-100 per meter) along its length, is delivered to a selected measurement site suspended on a hoisting cable, and is driven into the sea floor. As an alternative, a free-falling special lance package can be thrown over the side of a ship. It will sink to the bottom and drive the lance into the mud. After the requisite temperature measurements are made, an acquatic release can be triggered, returning the temperature recording package back to the surface on buoyant spheres, and leaving the replaceable lance shell in the mud. A record is made of a time sequence of temperature measurements taken by each temperature measuring device, and selected ones of these measurements are telemetered acoustically to a surface vessel, where they are used to check factors such as whether the lance was driven deep enough or straight enough, whether the telemetered measurements indicate suspicious or invalid data, whether the initial choice of intervals between measurement sites needs to be revised and, if so, by how much. After enough measurements have been taken to allow a correction to be made in accordance with the invention for transient effects such as heat build-up due to the friction in driving in the lance, while typically takes 5-20 minutes depending on the type of lance and the formation, the lance is moved to the next selected site, and the procedure is repeated until all sites have been serviced.
The lance is then retrieved, and the recorded temperature measurements are processed to find, for each depth at each site, an equilibrium temperature approaching the temperature which the measuring device would have recorded had it been left in place for a long time in a formation at the same temperature. Because the temperature measurement devices are so closely spaced, the record of equilibrium temperatures versus depth at a given site can be considered for practical purposes a nearly continuous temperature gradient.
It has been found important for this invention to account for the heat propagating down into the sea floor due to long term (seasonal) bottom water temperature variations. To this end, the effect of these variations is projected downwardly to find what part of the equilibrium temperature measured at a given depth for a given measurement site is due to those long term bottom water temperature variations.
If the temperature gradient for a given site, after having been corrected for the effect of these long term bottom water temperature variations, is substantially linear with depth, this is used as an indication that heat propagates through the surface formation only or mostly by conduction, and heat flow at the site can be found as a product of the so corrected temperature gradient and the thermal conductivity of the formation (which has been measured from core samples or is otherwise known). If the corrected temperature gradient is not a linear function of depth and is a curve which is substantially on one side of a straight line, this is used as an indication that there is significant advection (vertical flow of fluids through the formation at the site), and the heat flow at the site can be found as a function of the fluid velocity through the formation, the formation density, the heat capacity of the saturated medium, the top and the bottom temperatures measured at the site and the Peclet number.
The so-determined heat flow at the respective sites can be plotted to produce a heat flow map which, in accordance with the invention, can indicate the location of nearby subsurface hydrocarbon deposits. In addition, the otherwise available geological information can be used to construct a geological model of the formation below the sea floor, and this model can be compared with the map of the heat flow at the sea floor and modified until a satisfactory fit is observed between the model and the heat flow map. The sea floor temperatures can then be projected downwardly into the modelled formation, with corrections for factors such as conductivity variations with depth, sedimentation or erosion at the sea floor and migration of the hydrocarbon, to determine if the indicated hydrocarbons have matured at the right temperature for long enough to be commercially useful. Other corrections, such as for thermal refraction and the sea floor topography, can also be used in accordance with the invention.
A significant aspect of the invention is the recognition that temperature measurements taken over a depth of only a few meters by a driven-in lance rather than in a predrilled deep borehole, if taken in the indicated manner can be used in accordance with the invention to unscramble the signal of interest--sea floor heat flow due to migration of hydrocarbons in the recent geological past--from disturbing influences which can be much greater in magnitude. Another important aspect is the realization that surface heat flow measurements, whether taken at the sea bottom or on dry land, can be projected downwardly, with accounting for the relevant disturbing influences, so as to estimate the temperature and the time at which hydrocarbon deposits have evolved, so as to find whether the temperature history of such evolution suggests commercially useful hydrocarbon deposits. Other important aspects of the invention will become apparent from the detailed description below.